When wafers used as a substrate in the production of semiconductors are cleaned, the wafers are cleaned with various chemicals and subsequently rinsed with ultrapure water. The rinsing of wafers is performed using ultrapure water that is high-purity water from which impurities have been removed at a high level.
However, even the ultrapure water used for cleaning wafers may still contain trace metals. The trace metals contained in ultrapure water may adhere to wafers when the wafers are cleaned. Consequently, the wafers may become contaminated with the metals.
In order to reduce the metal contamination of products in the product-cleaning step, in the production of ultrapure water, commonly, several attempts have been made to increase the purity of ultrapure water by, for example, the refining of a deminar, the use of low-elution piping members or charged UF membranes, special pipe cleaning, or any combination thereof. However, all the above approaches increase the costs for an ultrapure water production facility. In addition, applying any of the above approaches to an already-existing facility may disadvantageously increase the amount of downtime required by the reconstruction work.
It is described in PTL 1 and PTL 2 that polystyrene sulfonic acid, a polystyrene-based quaternary ammonium salt, and the like contained in ultrapure water may contribute to the contamination of wafers and, therefore, it is necessary to reduce the contents of the above substances in ultrapure water.
In PTL 3, an agent for cleaning electronic materials which includes a quaternary ammonium salt (B) of an organic acid (A) is described. It is described in PTL 3 that particles can be removed without degrading the surfaces of wafers with an alkali metal. It is described in PTL 3 that, specifically, cleaning can be performed at a high level by preventing the particles removed from the cleaning target during cleaning from adhering again onto the cleaned surface of the target. However, there is no mention of a reduction in contamination caused by trace metals contained in the ultrapure water.
PTL 4 discloses a cleaning agent used for removing particles, the cleaning agent including a salt of polystyrene sulfonic acid as a surfactant. Similarly to PTL 3, there is no mention of a reduction in contamination caused by trace metals contained in the ultrapure water.                PTL 1: Japanese Patent Publication 2003-334550 A        PTL 2: Japanese Patent Publication 2004-283747 A        PTL 3: Japanese Patent Publication 2007-335856 A        PTL 4: Japanese Patent Publication 2014-141668 A        